Interference cancellation equipment

ABSTRACT

An interference cancellation method for use in a communications system in which a plurality of communication paths are arranged to transmit and receive respective analogue signals comprises effecting an initialization phase to calculate, for each path, the interference effects of the signals transmitted on that path on the signals received on each of the other paths, and storing for each a plurality of weighting factors representing the interference on each of the other paths respectively. During transmission, said weighting factors are used to generate from the transmitted signal on one of the said paths and to apply said cancellation signals to signals received on said other paths, thereby to cancel the interference effect of said transmitted signals.

[0001] This invention relates to interference cancellation equipmentuseful, for example, in cancelling interference in digital subscriberlines from central offices over twisted copper pair communication lines.

[0002] Digital subscriber line technology is a broadband datatransmission technique capable of being implemented on existing andfuture telecommunications networks. Digital subscriber line technologycan be implemented on telecommunications networks which rely on twistedcopper wire pairs to carry signals between a central office and a numberof end users and/or within the central office itself. In suchtelecommunications networks multiple insulated copper wire pairs arebundled together in a cable binder along portions of their route. Datagenerated digitally is converted into modulated analogue signals fortransmission. Conversely, received analogue signals are converted intodigital data.

[0003] One problem with such arrangements is that the signalstransmitted through a given twisted pair in the form of modulatedanalogue waveforms interfere with signals in adjacent twisted pairs.This form of interference is called crosstalk and it causes problemsbecause it makes it difficult to interpret the received analogue signalssubject to crosstalk. That is, crosstalk from other twisted pairs causesproblems in correctly demodulating and decoding the signal when it isreceived. Cross talk is most problematic when strong transmitted signalsin adjacent twisted pairs are of comparable frequency to the weakreceived signal.

[0004] In existing systems, crosstalk poses limitations on the cablelength, which affects received signal power due to attenuation.

[0005] Two different types of crosstalk degrade signal quality intwisted pair bundles. They are referred to as near end crosstalk (NEXT)and far end crosstalk (FEXT). Near end crosstalk is the most significantbecause the power of the transmitted signal is at its greatest, whilethe received signals have been attenuated during transit from the farend.

[0006] According to one aspect of the present invention there isprovided an interference cancellation method for use in a communicationssystem in which a plurality of communication paths are arranged totransmit and receive respective analogue signals, the communicationpaths being such that signals transmitted on one path interfere withsignals received on another path, the method comprising: effecting aninitialisation phase to calculate, for each path, the interferenceeffects of the signals transmitted on that path on the signals receivedon each of the other paths, and storing for each path a plurality ofweighting factors representing the interference on each of the otherpaths respectively; and during transmission, using said weightingfactors to generate from the transmitted signal on one of said paths, acancellation signal for each of the other paths and applying saidcancellation signal to signals received on said other paths thereby tocancel the interference effect of said transmitted signal.

[0007] According to another aspect of the present invention there isprovided interference cancellation equipment for use in a communicationssystem in which a plurality of communication paths are arranged totransmit and receive respective analogue signals, the communicationpaths being such that signals transmitted on one path interfere withsignals received on another path, the circuitry comprising: atransmitted data store for holding, for any of the paths, respectivesequences of digital data transmitted on those paths; a processor formonitoring received digital data in an initialise phase of operation tocalculate, for each path, the interference effects of the signalstransmitted on that path on the signals received on each of the otherpaths and for generating weighting factors; and a weighting factor storefor holding for each path a plurality of weighting factors representingthe interference on each of the other paths, respectively, wherein theprocessor is operable during transmission to use said weighting factorsto generate from the transmitted signal on one of said paths, acancellation signal for each of the other paths and to apply saidcancellation signal to signals received on said other paths thereby tocancel the interference effect of said transmitted signal.

[0008] According to another aspect of the present invention there isprovided a method of setting up a crosstalk information table in acommunications system comprising a plurality of communication paths, themethod comprising, for a first of said paths, transmitting apredetermined signal only on said path and detecting the received signalon each of the other paths in the absence of any other transmissioncorrelating the received signal on each path with the transmitted signalon the first path and calculating a crosstalk weighting factor bydetermining the ratio of the received signal power with the transmittedpower at a certain delay, storing the crosstalk weighting factor eachother path against the first path in the crosstalk information table,and repeating the steps for each of the second and subsequent paths.

[0009] The invention also provides a communication system comprising aplurality of twisted wire pairs each arranged to transmit and receiverespective analogue signals and being arranged in a common cable housingand an interference cancellation circuit which comprises: a transmitteddata store for holding, for any of said twisted pairs, respectivesequences of digital data transmitted on that pair; a processor formonitoring received digital data in an initialise phase of operation tocalculate for each pair the interference effects of the signalstransmitted on that pair on the signals received on each of the otherpairs and for generating weighting factors; and a weighting factor storefor holding for each pair a plurality of weighting factors representingthe interference on each of the other pairs respectively; wherein theprocessor is operable during transmission to use said weighting factorsto generate from the transmitted signal on one of said pairs acancellation signal for each of the other pairs.

[0010] The invention is particularly useful in cancellation of crosstalkwhere frequencies overlap on the transmit and receive sides. Forexample, for a downstream frequency range of 26 kHz to 1104 kHz and anupstream frequency range of 26 kHz to 138 kHz, there is an overlapfrequency band where crosstalk may be a problem, that is 26 kHz to 138kHz. The present invention can be used to effect in reducing thecrosstalk in such a frequency range, in particular at the higher end. Ofcourse, it can also be used outside that frequency range to improvesignal quality.

[0011] A specific embodiment of the present invention will now bedescribed, by way of example only, with reference to the accompanyingdrawings in which:

[0012]FIG. 1 is a schematic illustration of a telecommunications networkin which digital subscriber lines can be implemented;

[0013]FIG. 2 is a schematic illustration of multiple twisted paircommunication paths between a central office location and a plurality ofrespective users;

[0014]FIG. 3 is a schematic illustration of preferred communicationsequipment for connecting to twisted pairs;

[0015]FIG. 4 shows schematically how the effects of crosstalk can becomputed; and

[0016]FIG. 5 outlines schematically a procedure for initialisation ofcommunications equipment; and

[0017]FIG. 6 is an example of a correlation plot.

[0018] The phrase “downstream” used herein refers to the direction ofcommunication from the central office network to the end user and thephrase “upstream” refers to the reverse direction.

[0019] Referring to FIG. 1, a telecommunications network comprises aninter-central office network 10 connecting several central offices 12,14, 16 by means of optical fibres. A first local access network LAN1 isshown branching from a remote terminal 18 connected to the centraloffice 14 to a plurality of end user terminals T₀, T₁, T_(n-1). Theremote terminal 18 is located separately from the central office 14 towhich it is connected. Remote terminals are intermediate distributionpoints located geographically closer to the end user's premises in orderto improve service reliability. The communication line from the centraloffice 14 to the remote terminal 18 consists of a bundle 15 of twistedpairs. The communication lines 17 from the remote terminal 18 to the enduser terminals T₀,T₁, . . . T_(n-1) consist of individually routedtwisted pairs. Each twisted pair comprises two copper wires which aretwisted together and from which a voltage differential is driven toconvey analogue signal information down the pairs.

[0020] A second local access network LAN2 shown branching from anotherof the central offices 16 serves another group of end user terminalsT^(|) ₀, T^(|) ₁ . . . T⁵¹ _(n-1). In the second local access network,the respective end user terminals are connected directly to the centraloffice by individual twisted pair communications 17.

[0021] Within the central office there is provided central officeequipment necessary to provide end users with the digital networkservices to which they subscribe. Within the central office, data isconveyed digitally and is subject to digital signal processing,switching etc. For communications outside the central office, digitaldata is fed to and from a plurality of line cards via a high speedbi-directional digital feed on the network side of the line card. Eachline card connects to a plurality of twisted pairs on the end user side.

[0022] The twisted pairs running from the line card are physicallybundled together to form a single larger cable 15 running towards theend users. Inside the bundle the twisted pairs are arranged adjacent oneanother until the larger cable reaches a point where they may beconveniently separated and routed to geographically distinct locationsof the individual end users. In North America, twisted pair bundlesconventionally contain 24 separate twisted pair lines. Twisted pairbundles in Europe and elsewhere typically contain 32 individual twistedpair lines. Bundles containing different numbers of twisted pair linesare possible.

[0023]FIG. 2 shows a line card 20 located in a central officeenvironment. The line card 20 carries modem circuitry 25 which isconnected to a bi-directional digital data bus 23 in the central office.The modem circuitry 25 is connected to an analogue front end (AFE) whichincludes digital to analogue conversion circuitry 27 as well astransceiver circuitry 29. The transceiver circuitry includes output linedrivers and incoming receivers which are connected to a plurality oftwisted pairs 22, 24, 26, 28 serving remotely located end users T₀, T₁,. . . T_(n-1). Although not all are shown, there are a total of ntwisted pairs, each representing a bi-directional analogue communicationchannel #0,#1,#2 . . . #(n-1). As mentioned above, in existing systems nequals 24 or 32 but n can equal any suitable number.

[0024] The modem circuitry 25 on the line card 20 carries digital databetween the central office network and the analogue front end The AFEconverts between the analogue and digital domains. In the analoguedomain inductive and/or capacitive couplings between twisted pairs causecrosstalk. These couplings are indicated on FIG. 2 by means of thearrows designated XT₁₀, XT₂₀, XT_((n-1)0), XT₂₁, XT_((n-1)1). A muchhigher number of couplings would exist if all of the twisted paircommunication lines were shown on the drawing.

[0025] The notation XT₁₀ means the crosstalk between pairs #0, #1. Thatis, a signal transmitted on pair #0 will induce a signal XT₁₀ in pair#1.

[0026]FIG. 3 shows further particulars of a line card 20 andinterference cancellation circuitry 21 which may be implemented toreduce the effects of crosstalk. For the sake of clarity, FIG. 3 shows amodem chip 30 together with the AFE circuitry associated with only onetwisted pair 22 (channel #0). Each channel has its own associated AFEcircuitry, but several channels may be driven from a common modern chip.Each twisted pair is associated with modem logic provided in a modemchip. The modem logic for each channel implements a transmit and areceive path. The modem chip 30 is connected to receive digital signalsfrom the bus 23 in the downstream direction and supplies modulateddigital samples to a digital to analogue conversion circuit 32 forconverting the digital samples into analogue signals for transmissionover the twisted pair. A driver circuit 34 drives appropriate voltagedifferentials on to the twisted pair 22. Generation of the modulateddigital samples from the digital data on bus 23 is discussed later.

[0027] Each receive path leading from a given twisted pair in theupstream direction includes a receiver interface 36, and an analogue todigital conversion circuit 38 for converting analogue waveforms receivedfrom the users into digital samples. The driver 34 and receiver 36 areconnected via a hybrid interface 35 to the twisted pair 22. The hybridinterface 35 combines/splits the downstream analogue signal from thedriver 34 and the upstream analogue signal to the receiver 36 from thetransmit pair 22. The digital to analogue converter 32, driver circuit34, receiver 36 and analogue to digital converter 38 are replicated asnecessary to serve the total number of twisted pair communicationchannels #0,#1,#2 . . . #(n-1), the modem chip 30 being capable ofprocessing communications on all of the twisted pair channels 20simultaneously. The ADC 38 includes a sampler for supplying digitalsamples at a predetermined rate.

[0028] The bus 23 could be a backplane or implemented as twounidirectional buses.

[0029] The interference cancellation circuitry 21 includes a controller100, a transmit history table 102 and a crosstalk information table 104.

[0030] In the downstream direction, the modem chip 30 processes digitaldata received from the inter-central office network and modulates it tocreate a digital sampled representation of the analogue signal. Thissample data is fed to the digital to analogue converter 32, the outputof which is supplied to the driver circuitry 34 to drive a voltagedifferential onto the twisted pair 22.

[0031] In the upstream direction, an analogue signal is received via thehybrid interface 35 at the receiver, from where it is supplied to theanalogue to digital converter/sampler 38 for conversion from analoguewaveforms into a stream of digital signal samples which are supplied fordemodulation by the modem chip 30 and further processing by additionalnetwork components on the upstream side of the line card.

[0032] Demodulation reliability is improved by removing an “echo” of thetransmitted signals from received signals. The echo tends to be formedin the analogue front end and mixes in with the received signal comingupstream. This alters the shape of the received waveform. Withoutsuitable processing, this reflection of the downstream analogue signalwould interfere with the received signal and so cause problems incorrectly demodulating the latter.

[0033] The preferred method of removing the echo is by creating a secondoutput signal, referred to herein as the echo cancellation (EC) signal.The echo cancellation signal is intended to model closely the echo whichwould normally be received as a reflection of the downstream transmittedsignal. The echo cancellation signal once created is output through asecond digital to analogue converter and then electrically subtractedfrom the raw upstream signal before it is received. Since the rawupstream signal will contain both the far end user transmission, and thenear end echo from the downstream transmission, the echo cancellationsignal cancels the echo. This leaves just the intended end usertransmission signal to be processed.

[0034] The present invention is not focussed on the echo cancellations,but is primarily concerned with removing crosstalk interference fromother twisted pairs.

[0035] The transmit history table 102 is a record of signals transmitteddownstream from the central office equipment along each of the twistedpair channels #0 to #(n-1). The transmit history table 102 is thus an onchip memory space for recording values representing the signal samplemost recently transmitted on a given communication channel. In mostapplications it is sufficient to store the last 20 to 50 digital samplestransmitted. Each stored data sample 110 is associated with a labelT_(x0), T_(x1), . . . T_(Xn-1) indicating which of the communicationchannels #0,#1,#2 . . . #(n-1) the stored transmission data relates to.The data is stored as multibit digital samples, with each sample beingstored as 16 bits in one example. It is not necessary to store 16 bits;any suitable number of bits could be used to represent each sampledepending on the quality of the system.

[0036] The crosstalk information table 104 is another on chip memoryspace which stores for each communication channel #0 to #(n-1)information on the relative magnitudes of the crosstalk signal levelsinduced between a given twisted pair and each of the remaining pairs ofthe twisted pair bundle. The crosstalk information table 104 is ineffect a matrix with the various rows and columns containing weightingcoefficients and delay information indicating respectively the relativemagnitudes of the electromagnetic coupling between adjacent twistedpairs in the bundle and the delay time which is a timing factor.

[0037] Referring to the top row of the crosstalk information table 104channel numbers #0 to #(n-1) corresponding to each of the n twistedpairs of the bundle are listed horizontally. The left hand column of thecrosstalk information table 104 also lists the channel numberscorresponding to each of the twisted pairs of the bundle. Stored valueswithin the crosstalk information table 104 include for each pair ofinteracting channels a weighting coefficient and a delay function. Thus,the values stored in respect of the coupling between channel #0 andchannel #1 are ω₀ ¹ and τ₀ ¹, whereas the values stored in respect of,for example, channel #2 and channel #n are ω₂ ^(n) and τ₂ ^(n).

[0038] The controller 100 controls operation of the chip. The followingdescribes only its actions relating to interference cancellation. Thecontroller 100 operates in an initialisation phase to set up thecrosstalk information table 104. Then in, operation, the controller 100uses information from the transmit data table 102 and the crosstalkinformation table 104 to compute the cumulative effects of near endcrosstalk on the signals being received. The crosstalk between adjacenttwisted pairs is then taken into account by subtracting a deliberatelygenerated crosstalk cancellation signal from the received signal at anappropriate point in time.

[0039] Normal operation will firstly be described with reference to FIG.4. The controller 100 processes a received signal on channel #x withcrosstalk information including the weight coefficient and delay timefor that channel. It examines the transmit data for each of the otherchannels and generates a crosstalk cancellation signal based on thetransmit data, the weighting factor and delay time.

[0040] An algorithm for generation of the crosstalk cancellation signalcould be implemented in a similar fashion to that which is used toproduce the echo cancelling signal. The echo cancelling signal forchannel #n may be produced as the following sum (inverted):

T _(xn(t)) *e _(n(t)) +T _(xn(t-1)) *e _(n(t-1)) + . . . T _(xn(t-p)) *e_(n(t-p))

[0041] Where p is the number of signal samples used in echocancellation. P may be for example 20-50 values for echo cancellation,where T_(xn(t)) is the transmit value for channel #n at time t ande_(n(t)) is the echo coefficient for channel n at time t. For crosstalkcancellation, fewer samples will normally suffice. In the followingexample one sample is used, although in practice 2 to 10 samples (or anysuitable number) may be used.

[0042] This mechanism can be extended to the production of a crosstalkcancellation signal. The above sum is repeated, but the transmittedvalue is that for an adjacent channel and the coefficient is for thecrosstalk from that channel. In that sum, there are coefficients for anumber of points in time for each channel.

[0043] In a simplified method, there is one coefficient (w) for use withthe transmitted sample delayed at time τ. That is the crosstalkcancellation signal is the sum of {[crosstalk weight coefficient (w)*transmitted sample from crosstalk channel delayed by (τ)] for eachcrosstalk pair}, i.e.:

Σ_(#o)=(w ¹ _(o) *T _(x1)(τ_(o) ¹))+(w _(o) ² *T _(X2) *T _(X2)(τ_(o)²))+ . . . (w _(o) ^(n-1) *T _(x(n-1))(τ_(o) ^(n-1)))

[0044] Where w_(o) ² is the crosstalk weighting coefficient from channel#n to channel #0 and T_(xn) is the transmitted sample τ_(o) ^(n) clockcycles previously. Whether using the simplified or more complexformulae, the sum is repeated for each output channel using thetransmitted value from that channel and the crosstalk coefficients fromthat channel to the received channel.

[0045] The sum is repeated for each output channel using the transmittedvalue from that channel and the crosstalk coefficients from that channelto the received channel.

[0046] The sum of all of these represents the value that should be putout for that sample to the second digital to analogue converter 37,because this represents the combined cancellation signal for the echoand crosstalk.

[0047] As described above, in the preferred embodiment, removing thecrosstalk signal contribution from the received signal is achieved bycombining the crosstalk cancellation signal with the echo cancellationsignal which is already generated as a second output signal. Oncecreated the combined crosstalk and echo cancellation signal is outputthrough the second digital to analogue converter, and then electricallysubtracted from the raw upstream signal coming from the end user. Thecancellation signal cancels the signal perturbations caused by thecombined effects of crosstalk and echoes. This leaves an a high qualitycustomer transmission signal to be processed via the analogue to digitalconverter 38, sampler 39 and modem 30.

[0048] The values stored in the crosstalk information table 104 on whichcomputation of the crosstalk cancellation signal relies can bedetermined and stored for predetermined periods of time. Provided thecable bundles/network layout are not rearranged the relative proximityof the twisted pairs inside them ought not to change appreciably and thecrosstalk information values do not need to be refreshed. Preferredembodiments determine the values to be stored in the crosstalkinformation table in an initialisation process each time the centraloffice equipment is powered up as will be explained below with referenceto FIGS. 5 and 6.

[0049]FIG. 5 shows steps of an initialisation process which may beperformed by central office equipment in order to determine the valuesto be stored in the crosstalk information table 104. Referring to thestep designated 500, the plurality of receiver circuits 36 on the linecard 20 are switched on when the card is initially powered up. The modem25 then performs a predefined sequence of transmissions over each of thevarious channels #0 to #n-1 in turn (see step 510). In the absence ofany far end transmission activity, the driver circuit 34 of the firstchannel #0 transmits a known signal sample. In step 520, receivers onevery other channel record the reflected crosstalk signal at the linecard. These receivers correspond to the receiver 36 of the first channel#0. For each of the other channels, the received signal is correlatedwith the signal transmitted on channel #0 to produce a correlation curveas shown in FIG. 6. The weighting coefficient ω for each channel #xindicating the relative sizes of the coupling are derived from theresults by taking the ratio of the received signal power on channel #xto the transmitted power or channel #0 at the correlation peak at delayτ. That delay τ is also stored.

[0050] Referring back to FIG. 5, the step 530 records the weightingcoefficients ω and the associated delay time τ in the crosstalkinformation table for use later during normal operation of the centraloffice equipment. Steps 500 to 530 are repeated by performingtransmissions on each of the remaining channels #2 to #n in turn.

[0051] In this way the undesirable effects of crosstalk occurring in thewiring bundles of central office systems, and also elsewhere within thecentral office system, can be eliminated or reduced. Preferredembodiments have particular advantages where a single chip processesmultiple digital subscriber lines together because information from eachchannel is shared with all other channels on chip very effectively.

[0052] It would be possible to hold a number of weighting factors atdifferent delays for each channel to “finetune” the cancellation atdifferent parts of the line.

[0053] Preferred embodiments afford particular advantages when asdescribed above with modems implemented as logical channels within onechip capable of processing multiple communication channels. It is alsopossible however for modems to be implemented as individual chips or asa multi-chip set capable of processing multiple communication channels.

[0054] In one modification, the crosstalk cancellation signal remains inthe digital domain and is digitally subtracted from the received signalafter it has passed through the analogue to digital conversioncircuitry.

[0055] In another modification, transmit data information correspondingto that stored in the transmit data table is retained temporarily inbuffers associated with the respective line driver circuits 34. In suchan embodiment it would be useful to buffer up approximately 20 to 50recently transmitted data samples.

[0056] In another modification, the delay time information usuallystored in the crosstalk information table is neglected and so does notneed to be stored. Instead, an “average” delay is used for all channels.While this compromises performance it simplifies the crosstalkalgorithm. That is the same delay sample (T_(xn)) is used for allchannels.

[0057] In another embodiment, an impulse response is computed from thereceived sample data and is subsequently stored as a function in thecrosstalk information table with an indication of the range of delayvalues to which it corresponds.

[0058] Methods implemented in the downstream can be applied in theupstream direction, for example by determining crosstalk at userterminals and applying a cancellation signal at the end user's modem, inparticular where the user terminal has a multi-channel modem andcancellation is between the channels of this modem.

[0059] Signal transmission over digital subscriber lines can employ manydifferent modulation techniques and it will be apparent that preferredembodiments can be applied irrespective of the particular modulationtechnique being used.

[0060] Implementations are not limited to any of the arrangements orconfigurations described herein. Specifically, the described embodimentmerely represents an example of a configuration which may be used toimplement the preferred method.

1. An interference cancellation method for use in a communicationssystem in which a plurality of communication paths are arranged totransmit and receive respective analogue signals, the communicationpaths being such that signals transmitted on one path interfere withsignals received on another path, the method comprising: effecting aninitialisation phase to calculate, for each path, the interferenceeffects of the signals transmitted on that path on the signals receivedon each of the other paths, and storing for each path a plurality ofweighting factors representing the interference on each of the otherpaths respectively; and during transmission, using said weightingfactors to generate from the transmitted signal on one of said paths, acancellation signal for each of the other paths and applying saidcancellation signal to signals received on said other paths thereby tocancel the interference effect of said transmitted signal.
 2. Aninterference cancellation method according to claim 1 wherein thecommunication paths are arranged as twisted wire pairs.
 3. Aninterference cancellation method according to claims 1 or 2 wherein inthe initialisation phase, the signal transmitted on said one path iscorrelated with the signals received on each of the other paths,respectively and wherein the weighting factor for each of the otherpaths is derived from the ratio of the transmission power of thereceived signals on each path to the power of the signal transmitted onthat path at the correlation peak.
 4. An interference cancellationmethod according to claim 1, 2 or 3 wherein in the initialisation phasea timing value is stored with each weighting factor representing theeffective relative delay of the interference signal on each path.
 5. Aninterference cancellation method according to claim 4 wherein, for eachpath, a set of weighting factors with associated respective delays isstored representing interference on each of the other paths.
 6. Aninterference cancellation method according to any preceding claimwherein said cancellation signal is applied in the digital domain.
 7. Aninterference cancellation method according to any of claims 1 to 5wherein the cancellation signal is applied in the analogue domain.
 8. Aninterference cancellation method according to any preceding claimwherein transmit signal data is stored for use in generating a crosstalkcancellation signal during transmission.
 9. An interference cancellationmethod according to claim 4 wherein during transmission the cancellationsignal is generated by using transmitted signals at the effectiverelative delay for that path.
 10. Interference cancellation equipmentfor use in a communications system in which a plurality of communicationpaths are arranged to transmit and receive respective analogue signals,the communication paths being such that signals transmitted on one pathinterfere with signals received on another path, the circuitrycomprising: a transmitted data store for holding, for any of the paths,respective sequences of digital data transmitted on those paths; aprocessor for monitoring received digital data in an initialise phase ofoperation to calculate, for each path, the interference effects of thesignals transmitted on that path on the signals received on each of theother paths and for generating weighting factors; and a weighting factorstore for holding for each path a plurality of weighting factorsrepresenting the interference on each of the other paths, respectively,wherein the processor is operable during transmission to use saidweighting factors to generate from the transmitted signal on one of saidpaths, a cancellation signal for each of the other paths and to applysaid cancellation signals to signals received on said other pathsthereby to cancel the interference effect of said transmitted signal.11. Interference cancellation equipment according to claim 10 whichcomprises digital to analogue conversion circuitry for conveying digitaldata to be transmitted into analogue signals.
 12. Interferencecancellation equipment according to claim 10 or 11 which comprisesanalogue to digital conversion circuitry for converting receivedanalogue signals into digital data.
 13. Interference cancellationequipment according to any of claims 10 to 12 which comprises modemcircuitry connected as an interface to transmit and receive digitaldata.
 14. A communications system comprising a plurality of twisted wirepairs each arranged to transmit and receive respective analogue signalsand being arranged in a common cable housing and an interferencecancellation circuit which comprises: a transmitted data store forholding, for any of said twisted pairs, respective sequences of digitaldata transmitted on that pair; a processor for monitoring receiveddigital data in an initialise phase of operation to calculate for eachpair the interference effects of the signals transmitted on that pair onthe signals received on each of the other pairs and to generateweighting factors; and a weighting factor store for holding for eachpair a plurality of weighting factors representing the interference oneach of the other pairs respectively; wherein the processor is operableduring transmission to use said weighting factors to generate from thetransmitted signal on one of said pairs a cancellation signal for eachof the other pairs.
 15. A method of setting up a crosstalk informationtable in a communications system comprising a plurality of communicationpaths, the method comprising, for a first of said paths, transmitting apredetermined signal only on said path and detecting the received signalon each of the other paths in the absence of any other transmissioncorrelating the received signal on each path with the transmitted signalon the first path and calculating a crosstalk weighting factor bydetermining the ratio of the received signal power with the transmittedpower at a certain delay, storing the crosstalk weighting factor eachother path against the first path in the crosstalk information table,and repeating the steps for each of the second and subsequent paths.